This invention relates generally to ink jet printers and, more particularly, to an electrical circuit for protecting charge electrodes used in such printers in the event that the electrodes become shorted to ground potential.
In ink jet printers, printing is accomplished by depositing tiny drops of ink on a print receiving medium so that a print character is formed by the collection of drops. An ink jet printer typically includes a print head which defines a fluid reservoir containing electrically conductive ink. An orifice plate mounted on the print head defines a plurality of orifices arranged in one or more rows with each of the orifices communicating with the fluid reservoir. Ink is forced under pressure through the orifices as a plurality of fluid filaments. The filaments elongate and break into streams of tiny ink drops due to mechanical stimulation of the orifice plate or pressure waves which are generated in the fluid reservoir. Accordingly, the print head generates streams of ink drops of substantially uniform size with substantially uniform spacing between the drops.
Charge electrodes are positioned beneath the orifice plate and adjacent to the tip ends of the fluid filaments. Electrical potentials are selectively applied to the charge electrodes to induce corresponding charges of opposite polarity on the drops as they separate from the filament tip ends. The drops then pass downwardly through an electrical deflection field with the drops being deflected by the field to trajectories dependent upon a number of factors which include the charge level carried by the drops. The drops are then either caught or deposited on the print medium at desired locations dependent upon the trajectories of the drops.
The charge electrodes have typically comprised orifices in a charge electrode plate constructed of electrically insulating material with the orifices being lined with a conductive material such as a thin gold film to form the charge electrodes. An alternate construction of the charge electrodes which facilitates start-up and shut-down of the ink jet printer is a notched plate of insulating material with the notches being lined with conductive material to serve as the charge electrodes. Since the formation of the ink filaments and drops require a period of time to stabilize to the small sized, uniformly spaced drops required, the notched electrode plate permits movement away from the orifices until stable operation to thereby prevent fouling of the charge electrodes.
Whatever the construction of the charge electrodes, problems can arise if the electrodes are inadvertently connected to ground potential, for example, by an accumulation of ink on the charge electrode plate. Such ground faults of the charge electrodes can lead to high current levels which can damage or destroy the conductive material of the charge electrodes.
One approach which has been taken to reduce ground fault problems in charge electrodes is shown in IBM Technical Disclosure Bulletin, Volume 19, No. 2, July 1976. A charge electrode plate is therein disclosed having a major portion of the plate coated with an insulating material to resist shorts to ground. While this configuration is an improvement, the exposed charge electrodes may still short to ground potential with possible damage to the charge electrode plate.
A second approach to the reduction of charge electrode damage due to inadvertent ground faults is shown in U.S. Pat. No. 4,035,812, issued July 12, 1977, to Van Breemen et al. and assigned to the same assignee as the present invention. Van Breemen et al. discloses the use of bulk resistive material, such as an epoxy, filled with conductive particles to form the charge electrodes or discrete resistors connected in series with the charge electrodes. The resistance of the bulk resistive material or discrete resistors limits current flow to the charge electrodes in the event of ground faults. Van Breemen et al., while effective for most charge electrode ground faults, may not protect against faults occurring toward the charge electrode power supply and may also entail construction problems due to the large number of charge electrodes which must be provided in many ink jet printers.
It is, thus, apparent that an improved arrangement is necessary to protect ink jet printer charge electrodes from damage due to inadvertent ground faults.